On Mac:
1. Go to /Library/Audio/Plug-ins/Components/ACE.component/Contents/Resources/Images/Panes,
   find "ACE_Back1.png", and move it to another location (so you can revert to the original background pane whenever you want)
2. Place the customized "ACE_Back1.png" in the Panes folder (without renaming it)

On PC:
1. The installer will place ACE.data next to the .dll, first locate that.
2. Go to ACE.data\Data\Images\Panes,
   find "ACE_Back1.png", and move it to another location (so you can revert to the original background pane whenever you want)
3. Place the customized "ACE_Back1.png" in the Panes folder (without renaming it)